The aim of this study was to develop a test system to evaluate the effectiveness of procedures for decontamination of respirators contaminated with viral droplets. MS2 coliphage was used as a surrogate for pathogenic viruses. A viral droplet test system was constructed, and the size distribution of viral droplets loaded directly onto respirators was characterized using an aerodynamic particle sizer. The sizes ranged from 0.5 to 15 mum, and the sizes of the majority of the droplets were the range from 0.74 to 3.5 mum. The results also showed that the droplet test system generated similar droplet concentrations (particle counts) at different respirator locations. The test system was validated by studying the relative efficiencies of decontamination of sodium hypochlorite (bleach) and UV irradiation with droplets containing MS2 virus on filtering facepiece respirators. It was hypothesized that more potent decontamination treatments would result in corresponding larger decreases in the number of viable viruses recovered from the respirators. Sodium hypochlorite doses of 2.75 to 5.50 mg/liter with a 10-min decontamination period resulted in approximately 3- to 4-log reductions in the level of MS2 coliphage. When higher sodium hypochlorite doses (> or =8.25 mg/liter) were used with the same contact time that was used for the dilute solutions containing 2.75 to 5.50 mg/liter, all MS2 was inactivated. For UV decontamination at a wavelength of 254 nm, an approximately 3-log reduction in the level of MS2 virus was achieved with dose of 4.32 J/cm(2) (3 h of contact time with a UV intensity of 0.4 mW/cm(2)), while with higher doses of UV irradiation (> or =7.20 J/cm(2); UV intensity, 0.4 mW/cm(2); contact times, > or =5 h), all MS2 was inactivated. These findings may lead to development of a standard method to test decontamination of respirators challenged by viral droplets.